This is a vestige from when the only types of group that a user could create themselves was a private group, as public groups had to be created by the h admin. This is no longer the case, so we should allow the user to leave any group they are a member of. The option to leave public groups is present on activity pages.
The decision on whether to show the "leave group" option for a group in the groups menu is based on whether the group is private or not. See https://github.com/hypothesis/client/blob/8eea228063158e1853873174e16f26660b1487bc/src/sidebar/helpers/groups.ts#L56.
This is a vestige from when the only types of group that a user could create themselves was a private group, as public groups had to be created by the h admin. This is no longer the case, so we should allow the user to leave any group they are a member of. The option to leave public groups is present on activity pages.